As well known to those skilled in the art, among a number of joints in the human body, the pelvis and the femur can be rotated relating to one another within a predetermined angle. To this end, between the pelvis and the femur, there is intervened a hip joint for connecting the pelvis and the femur with each other in a rotatable manner.
The hip joint may be adversely influenced by stand-up walking, or may be injured by inherited factors, due to excessive exercise or through an accident. In the case that the hip joint is adversely influenced or injured, pain is caused at a boundary region where the pelvis and the femur are connected with each other. In order to replace the injured hip joint and ensure smooth rotation of the hip, an artificial hip joint prosthesis is provided. The artificial hip joint prosthesis has mainly been researched at the West with the progress in medical science, and therefore, fabricated in conformity with a body structure of a Westerner who has a larger physique and bone size than an Oriental.
Hereafter, several of the conventional artificial hip joint prostheses disclosed in the art will be briefly described.
The conventional artificial hip joint prosthesis shown in FIG. 16 comprises a pelvis-contacting element 110 fastened to a pelvis, a stem 150 fastened to a femur, a head 120 integrally formed at a distal end of the stem 150, a flexible joint member 130 which is coupled to the pelvis-contacting element 110 and in which the head 120 is accommodated in a freely rotatable manner, and a release prevention member 160 for preventing release of the flexible joint member 130 from the pelvis-contacting element 110.
The release prevention member 160 is meshed with the flexible joint member 130 along a circumferential direction through engagement between prominences and depressions (see the section ‘A’). An inner surface of the pelvis-contacting element 110 is defined with a groove 170, and the release prevention member 160 is formed with a projection 161 which is engaged into the groove 170.
The conventional artificial hip joint prosthesis constructed as mentioned above suffers from defects in that, since the flexible joint member 130 and the release prevention member 160 are formed as separate component parts, not only the number of component parts is increased, but also appreciable wear may take place due to rotation occurring therebetween.
Also, because the flexible joint member 130 and the release prevention member 160 are decreased in thickness at a region wherein they are meshed with each other through engagement between the prominences and the depressions, when the artificial hip joint prosthesis is used for an extended period of time, the region cannot but be weakened. Further, when the release prevention member 160 and the flexible joint member 130 are meshed with each other, since a height of a release preventing configuration of the release prevention member 160 is substantial, the stem 150 integrally rotated with the head 120 is likely to come into collision with the release prevention member 160. Therefore, if this collision occurs, as the rotation of the head 120 is interfered with, inordinate force can applied to the pelvis-contacting element 110, whereby the possibility of the pelvis to be adversely influenced is increased. Moreover, even with the flexible joint member 130 inserted into the pelvis-contacting element 110, positional fluctuation occurs due to play existing between the release prevention member 160 and the pelvis-contacting element 110 and the flexible joint member 130 and play existing between the flexible joint member 130 and the head 120, so that collision may easily occur between respective component parts.
Another conventional artificial hip joint prosthesis as shown in FIG. 17 also has the flexible joint member 130a inserted into the pelvis-contacting element 110a. The flexible joint member 130a is formed with six coupling portions 160a through 160f which are separated one from another in the circumferential direction and each of which is defined with a groove 161a. 
In this type of conventional artificial hip joint prosthesis, because the six coupling portions 160a through 160f are formed separately one from another, flexibility of the flexible joint member 130a is increased. However, when it is necessary to disassemble the flexible joint member 130a from the pelvis-contacting element 110a, all of the six coupling portions 160a through 160f should be simultaneously and resiliently contracted radially inward. Hence, where it is necessary to perform an operation again for the hip joint after decoupling the flexible joint member 130a and the pelvis-contacting element 110a from each other, inconvenience is caused.
Furthermore, in order to ensure decoupling of the flexible joint member 130a from the pelvis-contacting element 110a, the groove 161a must be defined on each of the coupling portions 160a through 160f in the circumferential direction. Thus, when the head (not shown) is, inserted into a flexible joint member 130a of increased size and rotated, the stem (not shown) is apt to collide with the flexible joint member 130a. 
Still another conventional artificial hip joint prosthesis as shown in FIG. 18 also has the head 120b, which is integrally formed at the distal end of the stem 150b. After the flexible joint member 130b is inserted into the pelvis-contacting element 110b, the head 120b is inserted into the flexible joint member 130b along with a support piece 162 which is placed around the head 120b. Then, a fixed locking piece 160b is fitted between the pelvis-contacting element 110b and the support piece 162 to allow the support piece 162 to be biased against the head 120b and thereby properly support the rotation of the head 120b. By the cooperation of the fixed locking piece 160b with the support piece 162, release of the head 120b and flexible joint member 130b from the pelvis-contacting element 110b is prevented.
Nevertheless, the conventional artificial hip joint prosthesis having been just described above encounters a problem in that, after the head 120b is inserted into the flexible joint member 130b, there exists a space C between the flexible joint member 130b and the fixed locking piece 160b, in which the support piece 162 can be moved, whereby collision may still occur between the head 120b and the flexible joint member 130b. Also, since the support piece 162 should be separately prepared, the entire manufacturing procedure is complicated. Further, since the increased number of component parts, that is, the flexible joint member 130b, the fixed locking piece 160b and the support piece 162 must be assembled in the pelvis-contacting element 10b, assemblability is deteriorated.
The above-described conventional artificial hip joint prostheses additionally have a disadvantage in that, since sizes of the artificial hip joint prostheses are substantial, difficulties are encountered when installing them. In other words, because the region where ends of the pelvis and femur are positioned is narrow, if assembling and disassembling operations are made complicated, difficulties cannot but be encountered when installing the artificial hip joint prostheses. In addition, because the conventional artificial hip joint prostheses are initially developed for Westerners who have large physiques and bone sizes, they cannot be appropriately adapted to Orientals.
Besides, in each of the conventional artificial hip joint prostheses, since the play in which the head can be moved to and for exists, collision frequently occurs between the head and the flexible joint member, whereby drawbacks associated with abrasion and wear of the component parts may be caused.